Optical fibers are used in many fields including telecommunications, laser machining and welding, laser beam and power delivery, fiber lasers, sensors and medical diagnostics and surgery. They are typically made entirely from solid transparent materials such as glass and each fiber typically has the same cross-sectional structure along its length. The transparent material in one part (usually the middle) of the cross-section has a higher refractive index than the rest and forms an optical core within which light is guided by total internal reflection. We refer to such a fiber as a standard fiber.
Although the light is confined to the core in a standard fiber, the cladding plays an active part in the wave-guiding process because a guided mode will extend some distance into the cladding. The cladding is also important for a relatively new class of fiber devices, know as cladding-pumped fiber lasers and amplifiers. The fibers used in such devices have an inner core, in which signal light propagates as a single-mode, and which is doped with some active material, typically a rare earth element. The inner core is nested in a larger outer core, which is multimode at both signal and pump wavelengths. Typically, the inner core is nested off-center within the outer core, to improve the overlap between the core mode and the modes of the cladding. High-power multi-mode pump light can be introduced into the outer core with a high efficiency, and propagates down the fiber, being gradually absorbed by the rare earth element present in the inner core. The signal in the inner core is then amplified, forming an optical amplifier or, with appropriate feedback, a laser.
Evanescent fiber sensors and couplers based on standard fibers are known in the form of “D” fibers. The preform from which a “D” fiber is drawn is polished away on one side until the core is close to the surface of the fiber. The fiber is then drawn and the thin layer of cladding glass remaining adjacent to the core in the previously polished region is etched away over a short length of fiber. The evanescent field of light propagating in the fiber is thus readily accessible only over that short length.
In the last few years, a non-standard type of optical fiber has been demonstrated, called the photonic-crystal fiber (PCF). Typically, this is made from a single solid, and substantially transparent, material within which is embedded a periodic array of air holes, running parallel to the fiber axis and extending longitudinally, the full length of the fiber. A defect in the form of a single missing air hole within the regular array forms a region of raised refractive index within which light is guided, in a manner analogous to total-internal-reflection guiding in standard fibers. The effective refractive index of each region of the fiber may be calculated using the methods outlined in, for example, Birks et al, Opt. Lett 22 961 (1997). Another mechanism for guiding light is based on photonic-band-gap effects rather than total internal reflection. Photonic-band-gap guidance can be obtained by suitable design of the array of air holes (see, for example, Birks et al, Electron. Lett. 31 1941 (1995)). Light with particular propagation constants can be confined to the core and will propagate therein.
Photonic-crystal fiber can be fabricated by stacking, on a macroscopic scale, glass canes—some of which are capillaries—into the required shape and then holding them in place while fusing them together and drawing them down into a fiber. PCF has unusual properties such as the ability to guide light in a single-mode over a very broad range of wavelengths, and to guide light having a relatively large mode area which remains single-mode.
This invention relates to the writing of Bragg gratings in the core of PCF and the overcoming of difficulties that have been associated therewith.
The fabrication of many photonic devices has been achieved through exposure of transmissive and absorbing materials to intense laser radiation in order to change the optical properties of said materials. For example, UV-induced photo-sensitivity of germanium doped silica glasses has been exploited in order to create permanent refractive index changes in the photosensitive Ge-doped silica cores of single mode optical fibers and waveguides as opposed to the undoped cladding. By creating a spatial intensity modulation of the UV exposure either by using a two-beam interference technique as disclosed in U.S. Pat. No. 4,807,950 by Glenn et al. or by using a phase mask as disclosed in U.S. Pat. No. 5,367,588 by Hill et al., Bragg grating structures can be produced in the photosensitive core of the waveguide.
Bragg gratings in optical fiber and waveguides have developed into an important technology for wavelength division multiplexing (WDM) systems and other applications for fiber optic systems such as optical sensing because of the highly desirable optical characteristics the Bragg structures exhibit as well as the relative ease with which they can be fabricated. A large variety of optical devices have been fabricated using Bragg gratings in waveguides including optical add/drop multiplexing filters (OADM), gain flattening filters, band splitters and dispersion compensators.
Photonic crystal fibers (PCFs) or microstructured fibers, as described above, consisting of a periodic array of air holes that make up a cladding region about a solid core, represent a new class of waveguides with unique modal, dispersive and nonlinear properties that have found applications in a variety of optical fields. The coupling of these two technologies, by the fabrication of grating structures within PCFs, has received much attention recently because of the potential advantages of exploiting together the distinct strengths of each technology.
Gratings fabricated in PCF with a Ge-doped core using existing FBG fabrication techniques are described by B. J. Eggleton et al. in “Grating resonances in air-silica microstructured optical fiber,” Opt. Lett. 24, 1460 (1999). Modest strength gratings have also been made in standard all-silica PCF using phase masks and an ArF excimer and UV femtosecond pulse duration laser radiation.
Although these prior art Bragg gratings in photonic crystal fiber provide a useful function, they are known to suffer from some limitations in terms of the strength of the grating resonance that can be produced. In fact only modest refractive index modulations have been reported, i.e., ˜1×10−4 and in the case of photonic crystal fiber consisting of pure silica, the grating writing times in this PCF were prohibitively long; over 1 hour when high photonic energy Argon Fluoride UV excimer laser radiation or high intensity femtosecond UV radiation is used.
Unfortunately, with side exposure inscription of Bragg gratings in PCF, scattering of light by the cladding holes is deleterious, especially when the exposure wavelength is on the same order as the hole dimensions and hole spacing or pitch that make up the photonic crystal cladding region, or more precisely the exposure wavelength is resonant with the photonic band gap created by the hole dimensions and spacing. The resulting intensity of energy incident on the core region is greatly reduced. PCFs have been characterized and the band gaps for particular PCFs have been determined. E. C Magi et al. in a paper entitled Transverse Characterization of Tapered Photonic Crystal Fibers, Journal of Applied Physics Vol 96 No. 7, 1 Oct. 2004, disclose the characterization of PCF as a function of a taper diameter, and rotational orientation of the photonic crystal lattice of the taper with respect to the incident probe beam, however there is no suggestion of writing a structure in such a fiber as a function of a mismatch between the irradiating wavelength and the band gap of the PCF.
As a solution to this problem we have discovered that this effect can be mitigated by either using a fiber geometry with fewer intervening holes between the core and outer surface or by removing or effectively removing the holes altogether. We have further discovered that by tapering a photonic crystal fiber sufficiently, scattering of light that otherwise would have occurred when writing a grating into the PCF using IR light can be lessened or effectively eliminated so that a high contrast grating can be written into the PCF. The hole diameter and hole spacing is reduced however in the absence of hole collapse; the ratio of hole diameter to hole spacing is constant. The change in hole diameter and spacing shifts the photonic band gap generated by the cladding holes away from the exposure wavelength. Effectively removing the holes for the purpose of writing a grating in the core, can also be achieved by using an index matching fluid placed into the holes or voids in PCF so that the scattering effect can be substantially minimized. Alternatively, actual removal of the holes can be done by completely collapsing the holes by sufficiently tapering the PCF. Yet, an alternative and less preferred solution to the problem of scattering of incident radiation through the crystal cladding region, is to manufacture PCF that has hole dimensions and hole spacing or pitch that is not on the same order as the exposure wavelength, or more precisely with hole dimensions and pitch that do not produce a photonic band gap at the exposure wavelength. Stated differently, one using IR or UV light to irradiate a PCF should preferably select the PCF with hole spacing and pitch to produce a photonic band gap to be a significantly different from the exposure wavelength so as to sufficiently lessen unwanted scattering. Selection of such a hole diameter and pitch may not always be possible depending on the mode propagation requirements of the PCF.
It is an object of this invention to overcome the aforementioned limitations within the prior art systems for fabrication of Bragg gratings in photonics crystal optical fiber and waveguides by substantially lessening or effectively removing the scattering effects of cladding holes in photonic crystal optical fiber and waveguides allowing for the induction of large refractive index change in said photonic crystal optical fibers and waveguides using laser radiation.
It is a further object of this invention to provide a method of lessening the scattering effect of the incident radiation used for writing a grating in the core region of a photonic crystal optical fiber by changing an aspect of the PCF's response to the light by: injection of index matching fluid into the cladding holes of the photonic crystal fiber which is transmissive to the radiation but is index matched to the photonic crystal fiber material (typically silica); or by tapering the PCF suitably so as to narrow a portion of the fiber prior to irradiating the fiber with light so as to write a grating therein.